This invention relates to a driveshaft of an automobile, more particularly to a hybrid driveshaft having a stem simply formed in one piece while the stem transmits a required driving torque and has high fundamental natural bending frequency to avoid a resonant critical vibration. This invention also relates to a method for producing the hybrid driveshaft.
Prior arts in the technical field to which this invention is attributed are as follows: U.S. Pat. No. 5,261,991 issued on Nov. 16, 1993, and entitled xe2x80x9cComposite Tubular Elements and Methods of Fabricationxe2x80x9d; U.S. Pat. No. 5,127,975 issued on Jul. 7, 1992, and entitled xe2x80x9cMethod of Fabrication of Composite Reinforced Driveshaft Having End Fitting secured theretoxe2x80x9d; U.S. Pat. No. 4,171,626 issued on Oct. 23, 1979, and entitled xe2x80x9cCarbon fiber reinforced Composite Driveshaftxe2x80x9d; U.S. Pat. No. 5,222,915 issued on Jun. 29, 1993, and entitled xe2x80x9cSelf-destructing Coupling Assembly for Use in Propeller Shafts of Motor Vehiclesxe2x80x9d; U.S. Pat. No. 4,131,701 issued on Dec. 26, 1978, and entitled xe2x80x9cComposite Tubular Elementsxe2x80x9d; U.S. Pat. No. 4,173,670 issued on Nov. 6, 1979, and entitled xe2x80x9cComposite Tubular Elementsxe2x80x9d; U.S. Pat. No. 4,214,932 issued on Jul. 29, 1980, and entitled xe2x80x9cMethod for Making Composite Tubular Elementsxe2x80x9d; U.S. Pat. No. 4,272,971 issued on Jun. 16, 1981, and entitled xe2x80x9cReinforced Tubular Structurexe2x80x9d; U.S. Pat. No. 4,173,128 issued on Nov. 6, 1979, and entitled xe2x80x9cComposite Driveshaftxe2x80x9d; U.S. Pat. No. 4,967,617 issued on Nov. 6, 1990, and entitled xe2x80x9cComposite Shaft with Integral Drive Elementsxe2x80x9d; and U.S. Pat. No. 3,553,978 issued on Jun. 7, 1967, and entitled xe2x80x9cComposite Propeller Shaft Construction and Method of Makingxe2x80x9d.
The driveshaft so called propeller shaft is used to transmit a driving torque from a transmission to a differential gear in an automobile. In a rear wheel driving car or a four-wheel driving car, the driveshaft is as long as 1.5xcx9c2.0 m. Although the driveshaft is designed to withstand several thousand Nm, the driveshaft may be broken when it is subject to a resonant critical vibration at speed of revolution corresponding to its fundamental bending natural frequency. In order to avoid such a catastrophic fracture, the fundamental bending natural frequency of the driveshaft has to be more than 6,500 rpm.
The fundamental bending natural frequency of the driveshaft with universal joints at both ends is in proportion to a square root of its specific modulus (E/xcfx81), and is in inverse proportion to a square of its length. In order to raise the fundamental bending natural frequency of the driveshaft, therefore, a value of the specific modulus must be increased, or the length of the driveshaft has to be shortened. As for one piece driveshaft made of steel or aluminum, it is difficult to obtain a desirable value of fundamental bending natural frequency under the condition of the length reached to 1.5xcx9c2.0 m. Thus, a conventional steel driveshaft is often produced in two pieces. Since the steel driveshaft needs an additional universal joint to connect between two pieces, it is sophisticated to produce and assemble the driveshaft.
Meanwhile, a carbon-fiber-reinforced polymer composite material (hereinafter, called as xe2x80x9ccomposite materialxe2x80x9d) has specific modulus more than four times of that of steel or aluminum. Thus, the driveshaft having the length reached to 1.5xcx9c2.0 m can be reduced in a unitary member with the fundamental bending natural frequency of more than 6500 rpm using the composite material.
However, it needs high cost to produce a driveshaft using only a composite material because the composite material is expensive.
Therefore, this invention is to propose a hybrid drive shaft in which a good torque transmission capability of metal and a high specific modulus of a composite material are well combined.
According to an aspect of this invention to accomplish the aforementioned aim, there is provided a hybrid driveshaft of an automobile. The hybrid driveshaft comprises a unitary cylindrical hybrid stem including a metal tube having high strength and a composite material layer having high specific modulus. Joints associated with a transmission and a differential gear are formed at both ends of the stem. The metal tube and the composite material layer are adhered to each other by co-curing under a high pressurized atmosphere, thereby, the metal tube and the composite material layer cooperate with each other to provide a good torque transmission capability and a high fundamental bending natural frequency of the driveshaft. Herein, the term xe2x80x9cco-curingxe2x80x9d means that there are simultaneously occurred several processes, that is, heating metal and composite material, lowering the viscosity of the resin of the composite material, and adhering the metal and the composite material by the resin.
According to another aspect of this invention, there is provided a method for producing a hybrid driveshaft of an automobile comprising a unitary cylindrical hybrid stem including a metal tube having high strength and a composite material layer having high specific modulus. The method comprises steps for forming joints at both ends of a metal tube, exerting an axial compressive force on the both ends of the metal tube to prevent it from being thermally expanded, stacking composite material on the metal tube, and co-curing the composite material by heating under high pressurized atmosphere so that the metal tube and the composite material are tightly adhered to each other. Preferably, the metal tube may be an aluminum alloy tube, and the composite material may be a unidirectional carbon fiber epoxy composite material. Moreover, the joints may comprise two yoke members, one of which is associated with a corresponding yoke formed in a transmission, and the other of which is associated with a corresponding yoke formed in a differential gear of an automobile. Furthermore, the axial compressive force may be exerted by a device comprising a fixed flange confronted with one end of the metal tube, a moving flange confronted with the other end of the metal tube, and a nut cooperating with a screw to force the moving flange toward the metal tube so that the metal tube is compressed between the fixed flange and the moving flange. It is preferred to accomplish mechanical and chemical roughness treatment on an outside surface of the metal tube before the stacking step of the composite material.
Other advantages and features of the present invention will become apparent from the following description, including the drawings and claims.